<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <!-- b.html: (http://www.domain2.com/b.html) -->
    <iframe
      id="iframe"
      src="http://www.domain1.com/c.html"
      style="display: none"
    ></iframe>
    <script>
      var iframe = document.getElementById("iframe");

      // 监听a.html传来的hash值，再传给c.html
      window.onhashchange = function () {
        iframe.src = iframe.src + location.hash;
      };
    </script>
    ``` c.html: (http://www.domain1.com/c.html) ```javascript
    <script>
      // 监听b.html传来的hash值
      window.onhashchange = function () {
        // 再通过操作同域a.html的js回调，将结果传回
        window.parent.parent.onCallback(
          "hello: " + location.hash.replace("#user=", "")
        );
      };
    </script>
  </body>
</html>
